Identification & utilization of novel drugs which selectively target key pathways in the immune system would greatly benefit the field of islet cell function. Issues of rejection, recurrence of autoimmunity, & primary non-function (early loss of islets from non-specific inflammatory events associated with intrahepatic transplantation) must all be considered when designing islet transplant strategies. An approach with significant potential to affect all these is blockade of the CD40-CD154 co-stimulatory pathway. Unprecedented, preliminary studies of islet transplantation in a non-human primate model clearly demonstrate the efficacy of anti-CD154 therapy for prolongation of islet allograft survival & maintenance of islet mass/metabolic control. Islet cell transplantation gives the ideal setting for testing & application of novel strategies for enhancement of allograft acceptance, because current immunosuppressive drugs are ineffective for a variety of recipients. The overall goal of this proposal is to incorporate anti-CD 154 therapy, alone or in combination with other immunotherapeutic interventions, into effective graft promoting strategies for application to clinical islet transplantation, with the ultimate goal of inducing donor specific tolerance, thus obviating the need for generalized immunosuppression. Specifically, we will 1) utilize a pre- clinical rhesus monkey model of islet allo-transplantation to a) determine the optimal timing and dosing of anti-CD 154, including optimization of induction, maintenance, & rescue therapy, b) determine if additional co- stimulatory blockade with anti-CD80 & CD86 further enhances islet survival, & c) determine if administration of selected &/or expanded donor bone marrow components will further promote graft survival under anti-CD 154 therapy, and possibly allow for the induction of donor-specific tolerance. To define graft acceptance/tolerance, 2) we will assess recipient anti-donor and anti-third party immunoreactivity, study the cytokine profile & cellular composition of recipient blood & tissues, & perform in vitro & in vivo experiments designed to alter graft acceptance/tolerance, thus providing support for a particular cytokine, cell, or molecule in maintenance of graft function.